1. Field of the Invention
The present invention relates to distribution and conservation of energy, and more particularly to an electrical power storage and conversion facility located at or near the intersection of electrical power grids.
2. Description of the Related Art
The United States and many other countries are partitioned into discrete geographic regions or "power grids" in which power plants coordinate with one another to provide electricity generating capabilities. These power grids evolved as power plants built transmission lines to serve remote markets. In most cases, transmission lines and loads at the boundaries between power grids are sparse and carry light loads, though this is not always the case. FIG. 1 is a United States map depicting areas covered by existing power grids or North American Electric Reliability Council (NERC) regions 100. Each of the NERC regions 100 encompasses a number of power companies, cooperatives and authorities. Many other countries are similarly divided into power grids.
Unlike power movement internal to the coordinated power grids, moving power from one power grid to another is very difficult. As noted, the transmission lines near boundary regions tend to be light, but there are commercial reasons as well. A power producer in one region is not excited by the prospect of cheap power from a neighboring region flooding into its region. The resulting lack of readily available and low-cost electricity in underdeveloped border regions results in stifled economic growth.
To further complicate potential power transfers, power generators in one region often produce power out of phase with that in the surrounding regions. For inter-region power transfer, these mismatches give rise to the necessity to convert the AC power to DC power, and then to reconvert the DC power back to AC power with the correct phase and frequency. Typical equipment includes a large motor and converter in the sending region (or power grid) and an inverter and generator in the receiving region. U.S. Pat. No. 5,561,597, entitled "COMPACT AND EFFICIENT TRANSFORMERLESS POWER CONVERSION SYSTEM" describes a technique for directly transferring power from one polyphase system to a second polyphase system without the use of a DC link. However, the system provides no energy storage capabilities.
Equipment associated with present conversion techniques is expensive (often reaching the $30-40 million range) and inefficient in that it only allows energy to move across a border between power grids as it is consumed in the receiving power grid. Since regional loads and prices tend to peak concurrently, economic benefit is minimized.
Referring briefly to FIG. 2, a load distribution chart illustrating a typical weekday electrical load profile in a given power grid is shown. The power generating capabilities of the power grid are constructed such that the majority of power demand (area 102) can be satisfied by baseload electrical power generation units, which typically take the form of coal or nuclear power plants. During periods of peak demand, which usually occur around midday, the difference between the power demand and power supplied by the baseload electrical power generation units is satisfied by cycling units (area 104) and/or peaking units (area 106) such as combustion turbines. This is significant from a cost standpoint because the power supplied by cycling and/or peaking units is much more expensive than the power supplied by the baseload electrical power generation units.
In the system depicted by FIG. 2, the baseload electrical power generation units are able to satisfy approximately 50% of peak load when operating at their highest load factor. As can be seen (areas 108), however, the baseload electrical power generation units of a typical power grid may not operate at full capacity during a significant portion of the day. Operating the baseload electrical power generation units at full capacity is desirable because the unit cost of electricity produced during such periods can be much less than the unit cost of electricity produced when the baseload units are operating at less than full capacity.
Various systems and methods for storing the relatively inexpensive off-peak energy within the confines of a power grid are known. For example, U.S. Pat. No. 4,849,648 to Longardner, entitled "Compressed Gas System and Method" and hereby incorporated by reference, discloses compressed air energy storage satellite facilities or tanks within an individual electrical power grid. The satellite facilities are independent of geological formations and are described as being capable of increasing the load carrying capacity of an electrical power system without increasing the size of the baseload electrical power generation facility or of the power transmission lines.
More specifically, the system described in Longardner circulates a portion of the compressed air back through a compressor located in a gas flow circuit, causing turbulent flow in a series of tanks, thus slowing heat energy loss to the environment. A heat exchanger located in the circuit of the gas flow cools the gas while it is being stored, reducing the work needed to compress a given mass of gas into the tanks.
Similarly, U.S. Pat. No. 4,353,214 to Gardner, entitled "Energy Storage System for Electrical Utility Plant" and hereby incorporated by reference, illustrates a method for storing and retrieving surplus energy produced by electric utility plants. The disclosed system method is implemented by utilizing large, underground caverns for storing a pressurized gas used as both the energy storage medium and the working gas in an expansion turbine. The disclosed system is capable of use with a single unit generator/motor combination with disengageable clutch assemblies to couple the unit to the turbine or compressor, depending on the operation phase.
According to Gardner, gas is communicated from a low pressure cavern through the closed system and into a compressor. The compressor is driven by a motor or equivalent power means and is actuated by electrical output received from the utility with which the system is associated. The compressed fluid is then fed to a high pressure conduit which is coupled to a high pressure cavern, thereby converting electrical energy from the associated utility into potential energy.
Energy retrieval in Gardner is effected by releasing the compressed gas through a high pressure release conduit to a turbine and generator which collectively produce the electrical output. The heated gas is then mixed and reacted with one or more fuels in a heating chamber preliminary to entering an expansion turbine which drives the generator. The expanded gas from the turbine is returned to the low pressure cavern through a return conduit.
Similar systems for storing energy by using compressed air energy storage (CAES) systems are known. The following patents, each of which is hereby incorporated by reference, are exemplary of such systems: U.S. Pat. No. 4,124,805 to Jacoby, entitled "Pollution-Free Power Generating and Peak Power Load Shaving System" (employing a subterranean cavity in thermal communication with a geological heat source); U.S. Pat. No. 4,237,692 to Ahrens et al., entitled "Air Ejector Augmented Compressed Air Energy Storage System" (utilizing a plurality of underground reservoirs in conjunction with a variable geometry air ejector and gas turbine system); U.S. Pat. No. 4,275,310 to Summers et al., entitled "Peak Power Generation" (employing a turbine generator facility activated by the expansion of compressed air withdrawn from an underground air storage reservoir in a substantially isothermal system); and U.S. Pat. No. 5,537,822 to Shnaid et al., entitled "Compressed Air Energy Storage Method and System" (heating air removed from a CAES system by using an external low grade energy source). In all of these references, however, use of the disclosed energy storage system is shown with a single generation facility or power grid.